Mapping Census output areas with d3

25 Jan 2014 22:20 GMT

This weekend I've been experimenting with using d3 to map the demography of Parliamentary constituencies using Census output areas. Output areas are the smallest areas for which Census data is produced — each output area typically contains just a few hundred people. As the smallest piece of the Census jigsaw, output areas provide the highest resolution picture of how the population varies within a larger area, such as a constituency or local authority.

I've recently started work on some research looking at the geographical distribution of different groups within Parliamentary constituencies and I wanted to explore different ways it could be presented. It seemed like an ideal oportunity to get to grips with d3's mapping APIs and I thought I'd share some of the results. Let me stress, this is not the research I have been working on, just an early experiment in presenting data from the 2011 Census at output area level.

My first attempt is a map of Welsh language ability on Ynys Môn (Anglesey), the island off the north coast of Wales that's just across the Menai Strait from where I was born, and where my parents now live. The map shows the percentage of the population that speaks Welsh in each output area, as recorded at the 2011 Census.

Welsh language ability is self-described in the Census (the ONS doesn't follow up with a test), but assuming people are equally good or bad at judging their Welsh language skills on average, irrespective of where on the island they live, the data should indicate the areas where more or less of the population are able to speak Welsh. Looking at the map, Welsh language speakers are more frequently found towards the centre of the island.